library
advancedJava Platform Module System (JPMS)
Understand Java modules: module-info.java, exports, requires, and strong encapsulation.
The Java Platform Module System (JPMS, aka Jigsaw, Java 9) adds a higher-level encapsulation above packages. A module is a group of packages with a module-info.java that declares:
Modules = apartments in a building. Each apartment (module) has a front door (exports) and private rooms (internal packages). The building directory (module-info) says who can visit whom.
Key Concepts
1
requires — which modules this module depends on
exports — which packages are accessible to other modules
opens — which packages are accessible to reflection (but not compile-time code)
provides/uses — service provider interfaces (SPI)
2
Before JPMS, any public class was accessible from anywhere (split packages, classpath hell). JPMS enforces:
- Strong encapsulation: even public classes in non-exported packages are inaccessible
- Reliable configuration: missing dependencies are detected at startup, not at first use
- No split packages: two modules can't contain the same package
3
The JDK itself is modularized: java.base (always available), java.sql, java.net.http, etc. You can create custom runtime images with jlink containing only the modules you need.
4
Modulepath vs classpath: code on the modulepath respects module boundaries. Code on the classpath is in the 'unnamed module' which can access all exported packages.
5
Adoption has been slow for applications (most still use classpath), but the JDK modularization improved performance and security.